Molecular chaperones play key roles in protein folding, transport across membranes, and in response to stressful stimuli. It was previously thought that prokaryotes contained a single hsp70-class chaperone, DnaK, but a second form, Hsc66, was recently discovered in E. coli (1). The crystal structure of a complete, intact hsp70 has not yet been determined, but structures for the N-terminal ATPase domain of bovine hsc70, human hsc70, and E. coli DnaK have been described (2,3). Hsc66 is of particular interest because it exhibits less sequence similarity, ~40%, to other hsp70s than any member of this highly conserved class. We have recently crystallized the ATPase domain of Hsc66 (residues 2-380, Mr~40k), and this fragment exhibits ATPase activity comparable to full length protein. The crystals of the ATPase domain of Hsc66 are small (15x30x4005m)but they diffract to high resolution (~2.2E) using synchrotron radiation at SSRL.